not quite sure why you're insisting on a "new" crossmember like the one in the picture that you provided. That crossmember is riveted on, you can probably find a selection at the scrap yard, pop the rivets off, hand the part to your local powder coat outfit, who will mediablast it and coat it any color you want.....it'll be good as new....in fact better, because it won't be an overseas, form pressed reproduction part. It will be a real, restored, GM crossmember.
Otherwise, the part can be fabricated by any local and reputable sheet metal shop.
Porterbuilt makes high quality drop members, frame supports, and suspension parts for square bodies.....but they are nothing like OEM.